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Determination of zero shear viscosity of warm asphalt binders
Construction and Building Materials 23 (2009) 2080–2086
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Determination of zero shear viscosity of warm asphalt binders Szabolcs Biro a, Tejash Gandhi a,*, Serji Amirkhanian b a b
Clemson University, 2002 Hugo Drive, Clemson, SC 29634, USA Department of Civil Engineering, Clemson University, 110 Lowry Hall, Clemson, SC 29634, USA
a r t i c l e
i n f o
Article history: Received 31 October 2007 Received in revised form 26 August 2008 Accepted 26 August 2008 Available online 14 October 2008 Keywords: Warm asphalt Warm asphalt binders Zero shear viscosity
a b s t r a c t With the increasing awareness of the warm asphalt technology, it is imperative to study the properties of the binders containing the warm asphalt additives thoroughly, especially since not much research has been conducted on warm asphalt binder properties to date. Also, in the recent years, researchers have observed that the SHRP rutting parameter G*/sin d is not very effective in predicting the rutting performance of binders, especially in case of modified binders. Zero shear viscosity (ZSV) has been evaluated to determine its effectiveness in predicting the rutting behavior of asphalt binders. Thus, in this paper, the ZSV of five asphalt binders with and without the warm asphalt additives, AsphaminÒ and SasobitÒ, were calculated using the different models and test methods available in literature. From the test results, it was observed that the addition of the warm asphalt additives increased the ZSV of all the five binders used in this study. It was also observed that the different test methods gave different ZSV values, and that the selection of the test methods and the testing parameters are crucial parameters. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The Strategic Highway Research Program (SHRP) specifies G*/ sin d as a parameter for determining the rutting resistance of the binder. G* is the complex modulus of the binder, which is a measure of resistance to deformation when the binder is subjected to repeated pulses of shear stress. The d parameter is the time lag between the applied stress and the resulting strain in the binder. However, the SHRP specifications suggested the use of these two parameters at a fixed temperature and frequency of testing. Since the viscosity of the binder changes with temperature, and the elastic properties of the binder change with the frequency of loading, the above parameters have been suggested to be ineffective in capturing the rutting of asphalt pavements, especially when modified asphalt binders are used [2,23,24]. Also, most polymer modified binders behave as non-Newtonian fluids at temperatures around 60 °C, thereby raising questions about the interpretation of the results of viscosity tests. Additionally, the G*/sin d cannot capture the recovery of the binder due to the fact that this parameter cannot distinguish between total energy dissipated and energy dissipated in permanent flow [1]. This can be misleading as a binder could have a low G*/sin d value, and still have a low permanent deformation in the repeated creep recovery test. In recent years, zero shear viscosity (ZSV) is being given a lot of attention by many researchers as a possible measure for the rutting resistance of modified asphalt
* Corresponding author. Tel.: +1 864 656 6185; fax: +1 864 656 6186. E-mail addresses: [email protected] (S. Biro), [email protected] (T. Gandhi), [email protected] (S. Amirkhanian). 0950-0618/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2008.08.015
binders [4,20,21]. ZSV, a theoretical concept, is the viscosity measured in shear deformation, when the shear rate is approaching zero. It is a measure of the viscosity of a material, when a shear stress is acting on it at a shear rate of almost zero. At such low shear rates, the binders undergo deformation so slowly, that it can adapt continuously to maintain equilibrium, despite the total amount of shear being large. The ZSV is said to be an indicator of two rutting related binder characteristics, namely the stiffness of the binder, and the binder’s resistance to permanent deformation under long term loading [26]. With increasing awareness of the warm asphalt technology in the asphalt industry, several properties of warm asphalt should be investigated. While several studies have been conducted to study the performance of warm asphalt mixtures [3,12–15], the properties of warm asphalt binder containing the warm asphalt additives are not known in great detail. 1.1. Objective In this paper, the ZSV of warm asphalt binder (binder modified with the warm asphalt additives AsphaminÒ and SasobitÒ) is calculated using different models and methods. The primary purpose of calculating the ZSV of the warm asphalt binders was to evaluate the rutting potential of the binders. While earlier studies on the mixtures indicate that the addition of AsphaminÒ has no significant effect on the rutting potential of the mixtures [12,13], and addition of SasobitÒ significantly lowered the rutting depths in the mixtures [12,13], preliminary studies conducted at Clemson University indicated that the rheological properties of the binders
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with AsphaminÒ and SasobitÒ increased the stiffness in the binders after the addition of both the additives. Additionally, an attempt was made to compare the different models and methods available to calculate the ZSV of asphalt binders. 2. Materials and experimental procedures Binders from five different sources were selected for this study. The first binder was from a Venezuelan source, the second was from different sources blended together, the third from Texas, fourth from Canada and the fifth from the Rocky Mountains. All the binders were of PG 64-22 grade, and their properties are as shown in Table 1. Warm asphalt was prepared using two of the available commercial products. Process 1 involved the addition of AsphaminÒ, a chemical powder at specified concentration (0.3% by weight of mixture – a binder content of 6% was assumed, and the entire additive was added to the binder) followed by mixing with a stirrer to disperse the powder through out the binder. AsphaminÒ is a Sodium–Aluminum–Silicate which has been hydro thermally crystallized as a very fine powder. It contains about 21% crystalline water by weight and is added to the mixture at a rate of 0.3% by weight of the mixture. By adding it to the mixture at the same time as the binder, a very fine water spray is created as all the crystalline water is released, which causes volume expansion in the binder, thereby increasing the workability and compatibility of the mixture at lower temperatures. It has been reported, by the manufacturer, that a reduction of about 25–30 °C has been observed. This specific property of AsphaminÒ is maintained over a long duration of time [8]. Process 2 involved addition of SasobitÒ, pellets at specified concentration (1.5% by weight of binder) followed by mixing for 5 min in a shear mixer to achieve consistent mixing. SasobitÒ is a long chain aliphatic hydrocarbon (chain lengths of 40–115 carbon atoms) obtained from coal gasification using the Fischer–Tropsch process. The Fischer–Tropsch process is a catalyzed chemical reaction in which carbon monoxide and hydrogen are converted into liquid hydrocarbons of various forms. The melting point of SasobitÒ is around 85–115 °C. SasobitÒ forms a homogeneous solution with the base binder on stirring, and produces a marked reduction in the binder’s viscosity. Reductions of about 25–50 °C in the mixing and handling temperatures of the mixture have been reported by the producer. After crystallization, SasobitÒ forms a lattice structure in the binder which is the basis of the structural stability of the binder containing SasobitÒ [22].
3. Results and discussion Following the preparation of the warm asphalt binders, rheological tests such as viscosity/flow measurements, frequency sweep, creep, and multiple creep recovery tests, at 60 °C were conducted using a Bohlin Dynamic Shear Rheometer with a 25 mm diameter plate–plate geometry, and 1 mm gap. Fig. 1 shows the results of viscosity/flow measurements, frequency sweep, creep response and multiple creep recovery tests for binder 1 with and without the warm asphalt additives. Other binders followed similar trends. More detailed results of the rheological tests on the five binders are submitted for publication elsewhere [10]. From the figure, it was observed that the binders containing the inorganic additive AsphaminÒ underwent only minor or no changes compared to the base binders in terms of flow properties. However, the flow of the binders containing SasobitÒ, which is an aliphatic hydrocarbon, changed from Newtonian to shear thinning flow. The addition of the warm asphalt additives significantly increased the viscosities of the binders at 60 °C, compared to the base binders. While the increase in the viscosity due to the addition of AsphaminÒ was as a result of the mineral filling effect of the zeolite, SasobitÒ is an aliphatic hydrocarbon, which crystallizes in the binder at temperatures below 85 °C, thereby increasing the stiffness of the binders. In the studied frequency range of 0.01–100 Hz, SasobitÒ increased the complex modulus of the binders at any given frequency. AsphaminÒ also increased the complex modulus of the binders, but the increase was not as much as SasobitÒ. Binders containing SasobitÒ showed lower compliance compared to the base binders indicating that they are stiffer and more resistant to penetration at mid-range temperatures. Binders containing AsphaminÒ also showed lower compliance values compared to the base binders in most cases. When repeated creep recovery tests were performed, binders containing SasobitÒ showed significantly lower permanent deformation compared to the base binders. Binders containing AsphaminÒ also showed lower compliance values, but the reduction in the permanent deformation was different with different binders. Thus, it was concluded that since AsphaminÒ acts only as mineral filler after the initial foaming, the stiffening effect of the additive must be dependant on the binder properties. Additionally, the data from creep test, frequency sweep test and the repeated creep test were used to calculate the ZSV of the warm asphalt binders using different models available in the literature. The results are described in the following sections. 3.1. Prediction of ZSV from creep test
Table 1 Properties of virgin asphalts Property
Binder 1
Binder 2
Binder 3
Binder 4
Binder 5
Original binder Viscosity, Pa s (135 °C) G*/sin d, kPa (64 °C)
0.626 1.801
0.405 1.207
0.457 1.315
0.453 1.321
0.420 1.284
RTFO residue Mass loss, % (163 °C) G*/sin d, kPa (64 °C)
0.24 4.608
0.02 2.815
0.01 3.780
0.06 2.93
0.14 3.27
PAV residue G*sin d, kPa (25 °C) Stiffness (60), MPa (12 °C) m-Value (60) (12 °C)
2420 129 0.345
2970 183 0.311
1704 117 0.320
1400 108 0.326
2565 132 0.335
64-22 163– 170 150– 155
64-22 150– 155 139– 144
64-22 –
64-22 –
64-22 –
–
–
–
PG grade Mixing temperaturea, °C Compaction temperaturea, °C a
Information provided by supplier.
Static creep is defined as the slow deformation of a material measured under a constant stress. In the static creep test, a fixed shear stress is applied to the sample and the resultant strain is monitored for a predetermined amount of time. If the stress is applied for a sufficiently long duration of time, the deformation in the binder reaches a constant value, which corresponds to the steady state flow of the binder. The viscosity of the binder at this stage is known as the steady state viscosity or the ZSV. 3.1.1. Prediction of ZSV based on Burger’s model In this study, a load of 100 Pa was applied for 4 h so that the deformation in the binder reaches steady state. Based on the recommendations of Giuliani [11], the ZSV can be extrapolated from the equation of Burger’s model (Eq. (1)), by means of measuring the creep compliance J(t) during the final portion of the test, where the binder deformation is in the steady state
cðtÞ ¼
s0 G0
þ
s0 G1
tG1 s0 1 e g1 þ t
g0
ð1Þ
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1000
Viscosity, Pas
1000
100
10
1 0.001
Binder 1 Binder 1 + Asphamin Binder 1 + Sasobit 0.01
0.1
1
Elastic and Viscous modulus, Pa
Shear rate, 10 8 10 7 10 6 10 5 10 4 10 3 10 2 10 1 10 0 10 -1 10 -2 0.01
10
Binder 1 Binder 1 + Asphamin Binder 1 + Sasobit 100 0.001
100
0.01
sec-1
0.1 Shear rate,
G`-Binder 1 G``-Binder 1 G`-Binder 1 + Asphamin G``-Binder 1 + Asphamin G`-Binder 1 + Sasobit G``-Binder 1 + Sasobit
1
10
sec-1
2.0
Compliance, 1/Pa
Shear stress, Pa
10000
Binder 1 Binder 1 + Asphamin Binder 1 + Sasobit
1.5
1.0
0.5
0.0 0.1
1
10
100
0
50
100
Frequency, Hz 0.30
200
250
300
Binder 1 Binder 1 + Asphamin Binder 1 + Sasobit
0.25 Compliance, 1/Pa
150 Time, s
0.20 0.15 0.10 0.05 0.00 0
10 20 30 40 50 470 480 490 500 510 520 Time, s
Fig. 1. Rheological properties of binder 1 at 60 °C: (a) flow curves; (b) viscosity curves; (c) frequency sweep; (d) creep response and (e) repeated creep recovery.
where c is deformation, s0 is shear stress value before the strain step (in relaxation), G0 and G1 are shear modulus values of the springs, g1 is shear viscosity of an individual Maxwell or Kelvin–Voigt element, g0 is zero shear viscosity, t is testing time (s). Giuliani suggested [11] that when the binder has reached a steady state flow, only the viscous portion of the Burger’s model (t/g0) varies. According to the theoretical assumptions of this method, the ZSV of an asphalt binder can be calculated as per Eq. (2) using the test data for the last 15 min. Fig. 2 shows the ZSVs calculated using this method for the five binders with and without the warm asphalt additives
ZSV ¼
Dt 900 ¼ DJ Jf J15
ð2Þ
where J15 is the compliance, 15 min before the load is removed, Jf is the compliance measured at the end of the creep test, 900 is the time interval (s) between the two compliance readings. From the graph, it can be seen that the addition of AsphaminÒ and SasobitÒ increases the ZSV of all the five binders at 60 °C. Additionally, the addition of SasobitÒ to binder 2 causes a substantial increase in the ZSV of the binder. It is believed that the increase in the ZSV of the binders due to the addition of AsphaminÒ is as a result of the mineral filling effect of the zeolite, which after initial
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Original binder Original binder + Asphamin Original binder + Sasobit
1000 800 600 400
3.2. ZSV determination from frequency sweep
200
As an alternative to determine the ZSV from the creep test, it is also possible to perform an oscillation test to determine the ZSV of a binder [19]. In the frequency domain, ZSV is related to the loss compliance J00 (x) according to Eq. (4) [9].
0 1
2
3
4
5
Binder source
J 00 ðxÞ
Fig. 2. ZSV values for the five binders with and without warm asphalt additives calculated from creep test by the Burger’s model.
foaming remains un-dissolved in the binders. Similarly, it is hypothesized that when SasobitÒ is added to the binders, the paraffinic wax is in a crystalline state at 60 °C, thereby increasing the stiffness of the binders, and thus the ZSV of the binders at 60 °C. 3.1.2. Prediction of ZSV based on Carreau’s model Based on the suggestions from Binard [4], a shear stress was applied to the sample in a dynamic shear rheometer for a duration long enough that the deformation reaches a constant value, which corresponds to steady state flow. At this stage, the viscosity of the binder is the steady state viscosity or the zero shear viscosity. In this study, a shear stress of 100 Pa was applied for a period of 4 h, so that the deformation reaches a steady state. The data from the end of the test, where the binder had reached a steady state, was selected and fitted as per the Carreau’s model (Eq. (3)) using the software SIGMAPLOTÒ and the ZSV was determined for the warm asphalt binders. Fig. 3 shows the ZSVs of the five binders with and without the warm asphalt additives calculated using this method.
g ¼
g0 g1 ½1 þ ðK xÞ2 m=2
þ g1
ð3Þ
Z 0
1
x½Jde ð1Þ Jde ðtÞ cos xt dt ¼
1
ð4Þ
xg0
where J00 is the loss compliance, x is the oscillation frequency, g0 is the first Newtonian region viscosity (absolute viscosity), t is the time. Consequently, when the oscillation frequency tends to zero, the ZSV can be determined in accordance with Eq. (5). Fig. 4 shows the ZSVs of the five binders with and without the warm asphalt additives, calculated from the frequency sweep test
g0 ¼
1 G 0 ¼ xJ x sin d
ð5Þ
where G* is the complex viscosity and d is the phase angle. If a frequency sweep test is performed on a binder, different equations can be fitted to the frequency sweep curves [4]. In this study, four decades of frequencies (0.01–0.1; 0.1–1; 1–10; and 10–100) were run at the lowest possible strain. In order to calculate the ZSV, the following models were applied to the frequency sweep curves, allowing extrapolating the viscosity to a frequency of 0 Hz. 3.2.1. ZSV determination based on Cross/Williamson’s model The data from a frequency sweep test was fitted to the Cross/ Williamson’s model (Eq. (6)) using the software SIGMAPLOTÒ. The data was then extrapolated to a frequency of 0 Hz in order to determine the ZSV of the binders with and without the warm as-
1200
1200
Zero shear viscosity, Pas
where g* is complex viscosity, g0 is first Newtonian region viscosity (absolute viscosity), g1 is infinite shear viscosity, x is frequency (rad/s), K and m are material parameters. The results indicated that the warm asphalt additives increased the ZSV of the five binders at 60 °C. It was also noticed that the ZSV values calculated from the Carreau’s model were very similar to the values obtained from Burger’s model calculations. Thus, it can be seen that the two models used to calculate the ZSV of the five binders from the creep test are quite consistent.
Original binder Original binder + Asphamin Original binder + Sasobit
1000 800 600 400 200 0
Original binder Original binder + Asphamin Original binder + Sasobit
1000
Zero shear viscosity, Pas
Zero shear viscosity, Pas
1200
800 600 400 200 0
1
2
3
4
5
Binder source Fig. 3. ZSV values for the five binders with and without warm asphalt additives calculated from creep test by the Carreau’s model.
1
2
3
4
5
Binder source Fig. 4. ZSV values for the five binders with and without warm asphalt additives calculated from frequency sweep test.
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1200
g g1 g ¼ 0 þ g1 1 þ ðK xÞm
1000
ð6Þ
where g* is complex viscosity, g0 is first Newtonian region viscosity (absolute viscosity), g1 is infinite shear viscosity, x is frequency (rad/s), K is a material parameter with the dimension of time, and m is a dimensionless material parameter. 3.2.2. ZSV determination based on Cross/Sybilski’s model Similarly, the data from the frequency sweep tests were fitted to the Cross/Sybilski’s model (Eq. (7)) using the software SIGMAPLOTÒ. The ZSVs calculated by extrapolating the data to a frequency of 0 Hz are shown in Fig. 6.
g ¼
g0
where g* is complex viscosity, g0 is first Newtonian region viscosity (absolute viscosity), x in frequency (rad/s), K is a material parameter with the dimension of time, and m is a dimensionless material parameter. 3.2.3. ZSV determination based on Carreau’s model The Carreau’s model (Eq. (3)) can also be fitted to the data obtained from a frequency sweep test to calculate the ZSV of a binder. The Cross and the Carreau’s models are very similar and preferred methods to calculate the ZSV in many R&D departments in the polymer industry [17]. The major difference between the Carreau’s model and the Cross’ model is that the Carreau’s model forces the formation of a plateau at low frequencies, leading to a more pronounced curvature than the Cross’ model, resulting in smaller ZSVs compared to the Cross’ model [4]. Fig. 7 shows the ZSVs of the five binders with and without the warm asphalt additives calculated using the software SIGMAPLOTÒ. It can be seen in Fig. 7 that the ZSV values of the five binders are slightly lower than the ZSV values obtained from the Cross’ model. From Figs. 4–7, it can be observed that the addition of AsphaminÒ and SasobitÒ increases the ZSVs of the binders. It was also observed that the ZSV values calculated from the different models are very similar and consistent. However, the ZSV values calculated from the frequency sweep test results are significantly lower than the ZSV values calculated from the creep test results. The ZSV values predicted by the Carreau’s model were different when the data
600 400 200
1
2
3
4
5
Binder source Fig. 6. ZSV values for the five binders with and without warm asphalt additives calculated from frequency sweep test by the Cross/Sybilski’s model.
1200 Original binder Original binder + Asphamin Original binder + Sasobit
1000 800 600 400 200 0 1
2
3
4
5
Binder source Fig. 7. ZSV values for the five binders with and without warm asphalt additives calculated from frequency sweep test by the Carreau’s model.
from the creep and frequency sweep tests were used. This suggests that the test method used to calculate the ZSV of asphalt binders also has an important influence on the ZSV values.
1200
Zero shear viscosity, Pas
800
ð7Þ
1 þ ðK xÞm
Original binder Original binder + Asphamin Original binder + Sasobit
0
Zero shear viscosity, Pas
Zero shear viscosity, Pas
phalt additives. The ZSVs of the binders calculated are shown in Fig. 5.
Original binder Original binder + Asphamin Original binder + Sasobit
1000 800 600 400 200 0 1
2
3
4
5
Binder source Fig. 5. ZSV values for the five binders with and without warm asphalt additives calculated from frequency sweep test by the Cross/Williamson’s model.
3.3. ZSV determination from repeated creep recovery test The repeated creep recovery test was conducted on all the five binders with and without the warm asphalt additives. The repeated creep recovery test simulates field conditions better as it applies a stress for a short duration of time and then leaves the material to recover for a longer duration of time, and repeats this several times. This in a way simulates vehicles passing on a pavement [4]. The test consisted of 52 cycles of loading with a stress of 10 Pa for 1 s, and recovery for 9 s. These testing parameters were based on the suggestions from the NCHRP 9-10 study [18]. Based on the recommendations [1,16], the time and strain data from the 50th and 51st cycles of the repeated creep recovery test were fitted to the Burger’s or four element model as per Eq. (8) using the software SIGMAPLOTÒ. The repeated creep recover test measures the total accumulated permanent strain in the binder after the 52 cycles, taking into consideration the elastic recovery after each cycle. The time and strain data were taken towards
S. Biro et al. / Construction and Building Materials 23 (2009) 2080–2086
the end of the test so that the binders have attained a steady state. The ZSVs calculated from the repeated creep recovery test using the Burger’s model are shown in Fig. 8. It can be seen from Fig. 8 that the five binders showed a similar trend as in case with other calculation methods.
cðtÞ ¼
s0 G0
þ
s0 G1
tG1 s0 1 e g1 þ t
ð8Þ
g0
where c is deformation, s0 is shear stress value before the strain step (in relaxation), G0 and G1 are shear modulus values of the springs, g1 is shear viscosity of an individual Maxwell or Kelvin– Voigt element, g0 is zero shear viscosity, t is testing time (s). 3.4. Comparison of different calculation methods From the results obtained in this study, it can be observed that the method of test to calculate the ZSV has a significant effect, however, ZSV values obtained from two different models using the data of a particular test are very similar. The determination of the ZSV from the creep test and the frequency sweep test (oscillation test) are described as the two most commonly used methods of calculating the ZSV of the binders by several researchers [5,6,20,21]. Since the ZSV determined by the creep test and the oscillation test theoretically measure the same property of the binder, the results have been compared by several researchers [7,25,27] and it was found that both the tests gave the same results for unmodified binders and binders with a low polymer content. However, the results of the two tests vary, especially for highly modified binders, because either the steady state has not reached in the binder within the testing duration of the creep test, or the frequency is not sufficiently low to obtain the lower frequency plateau in the viscosity curve in the oscillation test [19]. It was also reported that the Carreau’s and the Cross’ models were not suitable in predicting the ZSV of modified binders, as their curves do not reach a plateau at low shear rate or frequencies. In this study, it was observed that the ZSV values calculated form the creep and the frequency sweep tests were significantly different. The reason for this could be that the binders did not reach a steady state during the creep test, or the frequency of testing during the frequency sweep test was not low enough to obtain a plateau. Since in this study, a stress of 100 Pa was applied for duration of 4 h in the creep test on a binder not modified by any polymer, it is unlikely that the binder did not attain a steady state
1200 Original binder Original binder + Asphamin Original binder + Sasobit
Zero shear viscosity, Pas
1000 800 600 400 200 0 1
2
3
4
5
Binder source Fig. 8. ZSV values for the five binders with and without warm asphalt additives calculated from repeated creep recovery test by the Burger’s model.
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during the creep test. However, during the frequency sweep test, the lowest frequency of testing was 0.01 Hz, which may not be low enough to attain the plateau in the viscosity curve. Since the DSR used could not test at any lower frequencies, a higher temperature of testing can be used to calculate the ZSV from a frequency sweep test. It was decided not to test the samples at a higher temperature, as the ZSV is an indication of the rutting potential, and the high pavement service temperatures are about 60 °C. Calculation of the ZSV at temperatures higher than 60 °C may not have any significance if the pavement is not exposed to such temperatures in the field. Additionally, the authors believe that fitting data from a viscosity master curve can provide better results, as it can provide viscosity data at very low frequencies. Though the results of the ZSV obtained from the creep and frequency sweep tests were different, the trends observed with all the binders were very similar. Also, many factors such as creep testing time, frequency of loading, temperature of testing, software used to fit the test data to calculate the ZSV values, number of iterations used to fit the data, and many other factors affect the value of the ZSV calculated from any model using any test method. ZSV is only a theoretical concept, and there is no way to measure the absolute value, since there are so many approximations, and assumptions involved with the calculation of the ZSV. Thus, it is the opinion of the authors that the ZSV should be used only as a means of observing trends in the binders as a result of modification. In this study, it was observed that the addition of the warm asphalt additives increased the ZSV of the binder, irrespective of the test method and model used, which is consistent with the findings of another preliminary study at Clemson University, where the addition of the warm asphalt additives increased the stiffness of the binders and reduced the permanent deformation of the binders.
4. Conclusions From this limited study, the following can be concluded. It was observed that the addition of the warm asphalt additives increased the ZSV values of the five binders used in this study at 60 °C. In particular, the addition of SasobitÒ increased the ZSV of the binders the most in majority of the cases. This observation is consistent with an earlier study by the authors, where the addition of the warm asphalt additives increased the stiffness of the binders at 60 °C. It is believed that the increase in the ZSV values of the binders due to the addition of AsphaminÒ is as a result of the mineral filling effect of the zeolite, which after initial foaming remains un-dissolved in the binders. Similarly, it is hypothesized that when SasobitÒ is added to the binders, the paraffinic wax is in crystalline state at 60 °C, thereby increasing the stiffness of the binders, and thus the ZSV of the binders at 60 °C. ZSV values calculated from any particular test (creep, frequency sweep test, etc.) using different models are significantly similar to each other. ZSV values calculated from the same model using different test methods are significantly different. Thus, it can be seen that the test method used to calculate the ZSV, and the testing parameters used are important factors in calculating the ZSV of binders. Several factors like the type of test, test parameters used, software used to fit the test data to models, number of iterations performed to fit the test data, etc. affect the ZSV values calculated. Also, several assumptions and approximations are made in order to calculate the ZSV values for binders. As a result of this, the true ZSV value may never be obtained. Thus, the authors feel that the ZSV values should be used only as a means of observing trends in the binders as a result of modification.
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References [1] Bahia HU, Hanson DI, Zeng M, Zhai H, Khatri MA, Anderson RM. Characterization of modified asphalt binders in Superpave mix design. National Cooperative Highway Research Program Report 459. Washington: DC; 2001. [2] Bahia HU, Zeng M, Zhai H, Khatri. Superpave protocols for modified asphalt binders, 15th Quarterly Progess Report for NCHRP Project 9–10. Natioal Cooperative Highway Research Program. Washington: DC; 1999. [3] Barthel W, Marchand JP, Von Devivere M. Warm asphalt mixes by adding a synthetic zeolite. Eurovia.. [4] Binard C, Anderson D, Lapalu L, Planche JP. Zero shear viscosity of modified and unmodified binders. In: Proceedings of the 3rd Eurasphalt & Eurobitume congress, Vienna, 2004 – paper 236; 2004. p. 1721–33. [5] CEN TC 336. Bitumen and bituminous binders: determination of zero shear viscosity (ZSV) using a shear stress rheometer. Method 2: creep mode, draft 1.5, European Committee for Standardization; 2003. [6] Desmazes C, Lecomte M, Lesueur D, Phillips M. A protocol for reliable measurement of the zero shear viscosity in order to evaluate the anti-rutting performance of binders. In: Proceedings of the 2nd Eurasphalt & Eurobitume congress, Barcelona, Spain; 2000. [7] Dongré R, d’Angelo J. Refinement of SUPERPAVE High-Temperature Binder Specification Based on Pavement Performance in the Accelerated Loading Facility, 82nd Annual Meeting. Washington: Transportation Research Board; 2003. [8] Eurovia Services. [accessed January 2007]. [9] Ferry JD. Viscoelastic properties of polymers. Chichester (UK): John Wiley and Sons Inc.; 1980. [10] Gandhi TS, Amirkhanian SN. Laboratory investigation of warm asphalt binder properties – a preliminary investigation. In: MAIREPAV5 proceedings, Park City, UT, 8–10 August, 2007. [11] Giuliani F, Merusi F, Antunes I. Creep flow behavior of asphalt rubber binder – the zero shear viscosity analysis. In: Proceedings of the asphalt rubber 2006, Palm Springs, CA, 25–27 October, 2006. [12] Hurley G, Prowell B. Evaluation of Aspha-MinÒ for use in warm mix asphalt. NCAT report 05-04, Auburn; 2005. [13] Hurley G, Prowell B. Evaluation of SasobitÒ for use in warm mix asphalt. NCAT report 05-06, Auburn; 2005.
[14] Hurley G, Prowell B. Evaluation of EvothermÒ for use in warm mix asphalt. NCAT report 06-02, Auburn; 2006. [15] Hurley G, Prowell B. Evaluation of potential process for use in warm mix asphalt. J Assoc Asphalt Paving Technol 2006;75:41–90. [16] Marasteanu MO, Clyne T, McGraw J, Li X, Velasquez R. High temperature rheological properties of asphalt binders. In: TRB 2005 annual meeting CDROM; 2005. [17] Mezger TG. The rheology handbook – for users of rotational and oscillatory rheometers. Hannover: Vincentz Verlag; 2002. [18] NCHRP Project 9-10, Report 459. Characterization of modified asphalt binder in Superpave mix design.. [19] Nicholls C. Analysis of available data on validation of bitumen tests. Report on Phase I of the BiTVal Project, Forum of European National Highway Research Laboratories; 2006. . [20] Phillips MC, Robertus C. Binder rheology and asphalt pavement permanent deformation; the zero shear viscosity. In: Eurasphalt & Eurobitume congress, Straßbourg; 1996. [21] Rowe GM, D’Angelo JA, Sharrock MJ. Use of the zero shear viscosity as a parameter for the high temperature binder specification parameter. J Appl Asphalt Binder Technol 2002. [22] Sasol Wax. [accessed January 2007]. [23] Shenoy A. Model-fitting the master curves of the dynamic shear rheometer data to extract a rut-controlling term for asphalt pavements. ASTM J Test Eval 2002;30(2):95–102. [24] Smith BJ, Hesp S. Crack pinning in ashalt mastic and concrete:effect of rest periods and polymer modifiers on the fatigue life. In: Proceedings of the 2nd Euroasphalt & Eurobitume Congress, Bercelona; 2000. pp. 539-45. [25] Soenen H, Visscher J, De Vanelstraete A, Redelius P. The influence of thermal history on binder rutting indicators. Road Mater Pavement Des 2005;6(2):217–38. [26] De Visscher J, Soenen H, Vanelstraete A, Redelius P. A comparison of the zero shear viscosity from oscillation tests and the repeated creep test. In: Proceedings of the 3rd Eurasphalt & Eurobitume congress, Vienna, 2004, paper 153. [27] De Visscher J, Vanelstraete A. Practical test methods for measuring zero shear viscosity of bituminous binders. Mater Struct 2004;34:360–4.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Determination of zero shear viscosity of warm asphalt binders Szabolcs Biro a, Tejash Gandhi a,*, Serji Amirkhanian b a b
Clemson University, 2002 Hugo Drive, Clemson, SC 29634, USA Department of Civil Engineering, Clemson University, 110 Lowry Hall, Clemson, SC 29634, USA
a r t i c l e
i n f o
Article history: Received 31 October 2007 Received in revised form 26 August 2008 Accepted 26 August 2008 Available online 14 October 2008 Keywords: Warm asphalt Warm asphalt binders Zero shear viscosity
a b s t r a c t With the increasing awareness of the warm asphalt technology, it is imperative to study the properties of the binders containing the warm asphalt additives thoroughly, especially since not much research has been conducted on warm asphalt binder properties to date. Also, in the recent years, researchers have observed that the SHRP rutting parameter G*/sin d is not very effective in predicting the rutting performance of binders, especially in case of modified binders. Zero shear viscosity (ZSV) has been evaluated to determine its effectiveness in predicting the rutting behavior of asphalt binders. Thus, in this paper, the ZSV of five asphalt binders with and without the warm asphalt additives, AsphaminÒ and SasobitÒ, were calculated using the different models and test methods available in literature. From the test results, it was observed that the addition of the warm asphalt additives increased the ZSV of all the five binders used in this study. It was also observed that the different test methods gave different ZSV values, and that the selection of the test methods and the testing parameters are crucial parameters. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The Strategic Highway Research Program (SHRP) specifies G*/ sin d as a parameter for determining the rutting resistance of the binder. G* is the complex modulus of the binder, which is a measure of resistance to deformation when the binder is subjected to repeated pulses of shear stress. The d parameter is the time lag between the applied stress and the resulting strain in the binder. However, the SHRP specifications suggested the use of these two parameters at a fixed temperature and frequency of testing. Since the viscosity of the binder changes with temperature, and the elastic properties of the binder change with the frequency of loading, the above parameters have been suggested to be ineffective in capturing the rutting of asphalt pavements, especially when modified asphalt binders are used [2,23,24]. Also, most polymer modified binders behave as non-Newtonian fluids at temperatures around 60 °C, thereby raising questions about the interpretation of the results of viscosity tests. Additionally, the G*/sin d cannot capture the recovery of the binder due to the fact that this parameter cannot distinguish between total energy dissipated and energy dissipated in permanent flow [1]. This can be misleading as a binder could have a low G*/sin d value, and still have a low permanent deformation in the repeated creep recovery test. In recent years, zero shear viscosity (ZSV) is being given a lot of attention by many researchers as a possible measure for the rutting resistance of modified asphalt
* Corresponding author. Tel.: +1 864 656 6185; fax: +1 864 656 6186. E-mail addresses: [email protected] (S. Biro), [email protected] (T. Gandhi), [email protected] (S. Amirkhanian). 0950-0618/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2008.08.015
binders [4,20,21]. ZSV, a theoretical concept, is the viscosity measured in shear deformation, when the shear rate is approaching zero. It is a measure of the viscosity of a material, when a shear stress is acting on it at a shear rate of almost zero. At such low shear rates, the binders undergo deformation so slowly, that it can adapt continuously to maintain equilibrium, despite the total amount of shear being large. The ZSV is said to be an indicator of two rutting related binder characteristics, namely the stiffness of the binder, and the binder’s resistance to permanent deformation under long term loading [26]. With increasing awareness of the warm asphalt technology in the asphalt industry, several properties of warm asphalt should be investigated. While several studies have been conducted to study the performance of warm asphalt mixtures [3,12–15], the properties of warm asphalt binder containing the warm asphalt additives are not known in great detail. 1.1. Objective In this paper, the ZSV of warm asphalt binder (binder modified with the warm asphalt additives AsphaminÒ and SasobitÒ) is calculated using different models and methods. The primary purpose of calculating the ZSV of the warm asphalt binders was to evaluate the rutting potential of the binders. While earlier studies on the mixtures indicate that the addition of AsphaminÒ has no significant effect on the rutting potential of the mixtures [12,13], and addition of SasobitÒ significantly lowered the rutting depths in the mixtures [12,13], preliminary studies conducted at Clemson University indicated that the rheological properties of the binders
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with AsphaminÒ and SasobitÒ increased the stiffness in the binders after the addition of both the additives. Additionally, an attempt was made to compare the different models and methods available to calculate the ZSV of asphalt binders. 2. Materials and experimental procedures Binders from five different sources were selected for this study. The first binder was from a Venezuelan source, the second was from different sources blended together, the third from Texas, fourth from Canada and the fifth from the Rocky Mountains. All the binders were of PG 64-22 grade, and their properties are as shown in Table 1. Warm asphalt was prepared using two of the available commercial products. Process 1 involved the addition of AsphaminÒ, a chemical powder at specified concentration (0.3% by weight of mixture – a binder content of 6% was assumed, and the entire additive was added to the binder) followed by mixing with a stirrer to disperse the powder through out the binder. AsphaminÒ is a Sodium–Aluminum–Silicate which has been hydro thermally crystallized as a very fine powder. It contains about 21% crystalline water by weight and is added to the mixture at a rate of 0.3% by weight of the mixture. By adding it to the mixture at the same time as the binder, a very fine water spray is created as all the crystalline water is released, which causes volume expansion in the binder, thereby increasing the workability and compatibility of the mixture at lower temperatures. It has been reported, by the manufacturer, that a reduction of about 25–30 °C has been observed. This specific property of AsphaminÒ is maintained over a long duration of time [8]. Process 2 involved addition of SasobitÒ, pellets at specified concentration (1.5% by weight of binder) followed by mixing for 5 min in a shear mixer to achieve consistent mixing. SasobitÒ is a long chain aliphatic hydrocarbon (chain lengths of 40–115 carbon atoms) obtained from coal gasification using the Fischer–Tropsch process. The Fischer–Tropsch process is a catalyzed chemical reaction in which carbon monoxide and hydrogen are converted into liquid hydrocarbons of various forms. The melting point of SasobitÒ is around 85–115 °C. SasobitÒ forms a homogeneous solution with the base binder on stirring, and produces a marked reduction in the binder’s viscosity. Reductions of about 25–50 °C in the mixing and handling temperatures of the mixture have been reported by the producer. After crystallization, SasobitÒ forms a lattice structure in the binder which is the basis of the structural stability of the binder containing SasobitÒ [22].
3. Results and discussion Following the preparation of the warm asphalt binders, rheological tests such as viscosity/flow measurements, frequency sweep, creep, and multiple creep recovery tests, at 60 °C were conducted using a Bohlin Dynamic Shear Rheometer with a 25 mm diameter plate–plate geometry, and 1 mm gap. Fig. 1 shows the results of viscosity/flow measurements, frequency sweep, creep response and multiple creep recovery tests for binder 1 with and without the warm asphalt additives. Other binders followed similar trends. More detailed results of the rheological tests on the five binders are submitted for publication elsewhere [10]. From the figure, it was observed that the binders containing the inorganic additive AsphaminÒ underwent only minor or no changes compared to the base binders in terms of flow properties. However, the flow of the binders containing SasobitÒ, which is an aliphatic hydrocarbon, changed from Newtonian to shear thinning flow. The addition of the warm asphalt additives significantly increased the viscosities of the binders at 60 °C, compared to the base binders. While the increase in the viscosity due to the addition of AsphaminÒ was as a result of the mineral filling effect of the zeolite, SasobitÒ is an aliphatic hydrocarbon, which crystallizes in the binder at temperatures below 85 °C, thereby increasing the stiffness of the binders. In the studied frequency range of 0.01–100 Hz, SasobitÒ increased the complex modulus of the binders at any given frequency. AsphaminÒ also increased the complex modulus of the binders, but the increase was not as much as SasobitÒ. Binders containing SasobitÒ showed lower compliance compared to the base binders indicating that they are stiffer and more resistant to penetration at mid-range temperatures. Binders containing AsphaminÒ also showed lower compliance values compared to the base binders in most cases. When repeated creep recovery tests were performed, binders containing SasobitÒ showed significantly lower permanent deformation compared to the base binders. Binders containing AsphaminÒ also showed lower compliance values, but the reduction in the permanent deformation was different with different binders. Thus, it was concluded that since AsphaminÒ acts only as mineral filler after the initial foaming, the stiffening effect of the additive must be dependant on the binder properties. Additionally, the data from creep test, frequency sweep test and the repeated creep test were used to calculate the ZSV of the warm asphalt binders using different models available in the literature. The results are described in the following sections. 3.1. Prediction of ZSV from creep test
Table 1 Properties of virgin asphalts Property
Binder 1
Binder 2
Binder 3
Binder 4
Binder 5
Original binder Viscosity, Pa s (135 °C) G*/sin d, kPa (64 °C)
0.626 1.801
0.405 1.207
0.457 1.315
0.453 1.321
0.420 1.284
RTFO residue Mass loss, % (163 °C) G*/sin d, kPa (64 °C)
0.24 4.608
0.02 2.815
0.01 3.780
0.06 2.93
0.14 3.27
PAV residue G*sin d, kPa (25 °C) Stiffness (60), MPa (12 °C) m-Value (60) (12 °C)
2420 129 0.345
2970 183 0.311
1704 117 0.320
1400 108 0.326
2565 132 0.335
64-22 163– 170 150– 155
64-22 150– 155 139– 144
64-22 –
64-22 –
64-22 –
–
–
–
PG grade Mixing temperaturea, °C Compaction temperaturea, °C a
Information provided by supplier.
Static creep is defined as the slow deformation of a material measured under a constant stress. In the static creep test, a fixed shear stress is applied to the sample and the resultant strain is monitored for a predetermined amount of time. If the stress is applied for a sufficiently long duration of time, the deformation in the binder reaches a constant value, which corresponds to the steady state flow of the binder. The viscosity of the binder at this stage is known as the steady state viscosity or the ZSV. 3.1.1. Prediction of ZSV based on Burger’s model In this study, a load of 100 Pa was applied for 4 h so that the deformation in the binder reaches steady state. Based on the recommendations of Giuliani [11], the ZSV can be extrapolated from the equation of Burger’s model (Eq. (1)), by means of measuring the creep compliance J(t) during the final portion of the test, where the binder deformation is in the steady state
cðtÞ ¼
s0 G0
þ
s0 G1
tG1 s0 1 e g1 þ t
g0
ð1Þ
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1000
Viscosity, Pas
1000
100
10
1 0.001
Binder 1 Binder 1 + Asphamin Binder 1 + Sasobit 0.01
0.1
1
Elastic and Viscous modulus, Pa
Shear rate, 10 8 10 7 10 6 10 5 10 4 10 3 10 2 10 1 10 0 10 -1 10 -2 0.01
10
Binder 1 Binder 1 + Asphamin Binder 1 + Sasobit 100 0.001
100
0.01
sec-1
0.1 Shear rate,
G`-Binder 1 G``-Binder 1 G`-Binder 1 + Asphamin G``-Binder 1 + Asphamin G`-Binder 1 + Sasobit G``-Binder 1 + Sasobit
1
10
sec-1
2.0
Compliance, 1/Pa
Shear stress, Pa
10000
Binder 1 Binder 1 + Asphamin Binder 1 + Sasobit
1.5
1.0
0.5
0.0 0.1
1
10
100
0
50
100
Frequency, Hz 0.30
200
250
300
Binder 1 Binder 1 + Asphamin Binder 1 + Sasobit
0.25 Compliance, 1/Pa
150 Time, s
0.20 0.15 0.10 0.05 0.00 0
10 20 30 40 50 470 480 490 500 510 520 Time, s
Fig. 1. Rheological properties of binder 1 at 60 °C: (a) flow curves; (b) viscosity curves; (c) frequency sweep; (d) creep response and (e) repeated creep recovery.
where c is deformation, s0 is shear stress value before the strain step (in relaxation), G0 and G1 are shear modulus values of the springs, g1 is shear viscosity of an individual Maxwell or Kelvin–Voigt element, g0 is zero shear viscosity, t is testing time (s). Giuliani suggested [11] that when the binder has reached a steady state flow, only the viscous portion of the Burger’s model (t/g0) varies. According to the theoretical assumptions of this method, the ZSV of an asphalt binder can be calculated as per Eq. (2) using the test data for the last 15 min. Fig. 2 shows the ZSVs calculated using this method for the five binders with and without the warm asphalt additives
ZSV ¼
Dt 900 ¼ DJ Jf J15
ð2Þ
where J15 is the compliance, 15 min before the load is removed, Jf is the compliance measured at the end of the creep test, 900 is the time interval (s) between the two compliance readings. From the graph, it can be seen that the addition of AsphaminÒ and SasobitÒ increases the ZSV of all the five binders at 60 °C. Additionally, the addition of SasobitÒ to binder 2 causes a substantial increase in the ZSV of the binder. It is believed that the increase in the ZSV of the binders due to the addition of AsphaminÒ is as a result of the mineral filling effect of the zeolite, which after initial
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Original binder Original binder + Asphamin Original binder + Sasobit
1000 800 600 400
3.2. ZSV determination from frequency sweep
200
As an alternative to determine the ZSV from the creep test, it is also possible to perform an oscillation test to determine the ZSV of a binder [19]. In the frequency domain, ZSV is related to the loss compliance J00 (x) according to Eq. (4) [9].
0 1
2
3
4
5
Binder source
J 00 ðxÞ
Fig. 2. ZSV values for the five binders with and without warm asphalt additives calculated from creep test by the Burger’s model.
foaming remains un-dissolved in the binders. Similarly, it is hypothesized that when SasobitÒ is added to the binders, the paraffinic wax is in a crystalline state at 60 °C, thereby increasing the stiffness of the binders, and thus the ZSV of the binders at 60 °C. 3.1.2. Prediction of ZSV based on Carreau’s model Based on the suggestions from Binard [4], a shear stress was applied to the sample in a dynamic shear rheometer for a duration long enough that the deformation reaches a constant value, which corresponds to steady state flow. At this stage, the viscosity of the binder is the steady state viscosity or the zero shear viscosity. In this study, a shear stress of 100 Pa was applied for a period of 4 h, so that the deformation reaches a steady state. The data from the end of the test, where the binder had reached a steady state, was selected and fitted as per the Carreau’s model (Eq. (3)) using the software SIGMAPLOTÒ and the ZSV was determined for the warm asphalt binders. Fig. 3 shows the ZSVs of the five binders with and without the warm asphalt additives calculated using this method.
g ¼
g0 g1 ½1 þ ðK xÞ2 m=2
þ g1
ð3Þ
Z 0
1
x½Jde ð1Þ Jde ðtÞ cos xt dt ¼
1
ð4Þ
xg0
where J00 is the loss compliance, x is the oscillation frequency, g0 is the first Newtonian region viscosity (absolute viscosity), t is the time. Consequently, when the oscillation frequency tends to zero, the ZSV can be determined in accordance with Eq. (5). Fig. 4 shows the ZSVs of the five binders with and without the warm asphalt additives, calculated from the frequency sweep test
g0 ¼
1 G 0 ¼ xJ x sin d
ð5Þ
where G* is the complex viscosity and d is the phase angle. If a frequency sweep test is performed on a binder, different equations can be fitted to the frequency sweep curves [4]. In this study, four decades of frequencies (0.01–0.1; 0.1–1; 1–10; and 10–100) were run at the lowest possible strain. In order to calculate the ZSV, the following models were applied to the frequency sweep curves, allowing extrapolating the viscosity to a frequency of 0 Hz. 3.2.1. ZSV determination based on Cross/Williamson’s model The data from a frequency sweep test was fitted to the Cross/ Williamson’s model (Eq. (6)) using the software SIGMAPLOTÒ. The data was then extrapolated to a frequency of 0 Hz in order to determine the ZSV of the binders with and without the warm as-
1200
1200
Zero shear viscosity, Pas
where g* is complex viscosity, g0 is first Newtonian region viscosity (absolute viscosity), g1 is infinite shear viscosity, x is frequency (rad/s), K and m are material parameters. The results indicated that the warm asphalt additives increased the ZSV of the five binders at 60 °C. It was also noticed that the ZSV values calculated from the Carreau’s model were very similar to the values obtained from Burger’s model calculations. Thus, it can be seen that the two models used to calculate the ZSV of the five binders from the creep test are quite consistent.
Original binder Original binder + Asphamin Original binder + Sasobit
1000 800 600 400 200 0
Original binder Original binder + Asphamin Original binder + Sasobit
1000
Zero shear viscosity, Pas
Zero shear viscosity, Pas
1200
800 600 400 200 0
1
2
3
4
5
Binder source Fig. 3. ZSV values for the five binders with and without warm asphalt additives calculated from creep test by the Carreau’s model.
1
2
3
4
5
Binder source Fig. 4. ZSV values for the five binders with and without warm asphalt additives calculated from frequency sweep test.
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1200
g g1 g ¼ 0 þ g1 1 þ ðK xÞm
1000
ð6Þ
where g* is complex viscosity, g0 is first Newtonian region viscosity (absolute viscosity), g1 is infinite shear viscosity, x is frequency (rad/s), K is a material parameter with the dimension of time, and m is a dimensionless material parameter. 3.2.2. ZSV determination based on Cross/Sybilski’s model Similarly, the data from the frequency sweep tests were fitted to the Cross/Sybilski’s model (Eq. (7)) using the software SIGMAPLOTÒ. The ZSVs calculated by extrapolating the data to a frequency of 0 Hz are shown in Fig. 6.
g ¼
g0
where g* is complex viscosity, g0 is first Newtonian region viscosity (absolute viscosity), x in frequency (rad/s), K is a material parameter with the dimension of time, and m is a dimensionless material parameter. 3.2.3. ZSV determination based on Carreau’s model The Carreau’s model (Eq. (3)) can also be fitted to the data obtained from a frequency sweep test to calculate the ZSV of a binder. The Cross and the Carreau’s models are very similar and preferred methods to calculate the ZSV in many R&D departments in the polymer industry [17]. The major difference between the Carreau’s model and the Cross’ model is that the Carreau’s model forces the formation of a plateau at low frequencies, leading to a more pronounced curvature than the Cross’ model, resulting in smaller ZSVs compared to the Cross’ model [4]. Fig. 7 shows the ZSVs of the five binders with and without the warm asphalt additives calculated using the software SIGMAPLOTÒ. It can be seen in Fig. 7 that the ZSV values of the five binders are slightly lower than the ZSV values obtained from the Cross’ model. From Figs. 4–7, it can be observed that the addition of AsphaminÒ and SasobitÒ increases the ZSVs of the binders. It was also observed that the ZSV values calculated from the different models are very similar and consistent. However, the ZSV values calculated from the frequency sweep test results are significantly lower than the ZSV values calculated from the creep test results. The ZSV values predicted by the Carreau’s model were different when the data
600 400 200
1
2
3
4
5
Binder source Fig. 6. ZSV values for the five binders with and without warm asphalt additives calculated from frequency sweep test by the Cross/Sybilski’s model.
1200 Original binder Original binder + Asphamin Original binder + Sasobit
1000 800 600 400 200 0 1
2
3
4
5
Binder source Fig. 7. ZSV values for the five binders with and without warm asphalt additives calculated from frequency sweep test by the Carreau’s model.
from the creep and frequency sweep tests were used. This suggests that the test method used to calculate the ZSV of asphalt binders also has an important influence on the ZSV values.
1200
Zero shear viscosity, Pas
800
ð7Þ
1 þ ðK xÞm
Original binder Original binder + Asphamin Original binder + Sasobit
0
Zero shear viscosity, Pas
Zero shear viscosity, Pas
phalt additives. The ZSVs of the binders calculated are shown in Fig. 5.
Original binder Original binder + Asphamin Original binder + Sasobit
1000 800 600 400 200 0 1
2
3
4
5
Binder source Fig. 5. ZSV values for the five binders with and without warm asphalt additives calculated from frequency sweep test by the Cross/Williamson’s model.
3.3. ZSV determination from repeated creep recovery test The repeated creep recovery test was conducted on all the five binders with and without the warm asphalt additives. The repeated creep recovery test simulates field conditions better as it applies a stress for a short duration of time and then leaves the material to recover for a longer duration of time, and repeats this several times. This in a way simulates vehicles passing on a pavement [4]. The test consisted of 52 cycles of loading with a stress of 10 Pa for 1 s, and recovery for 9 s. These testing parameters were based on the suggestions from the NCHRP 9-10 study [18]. Based on the recommendations [1,16], the time and strain data from the 50th and 51st cycles of the repeated creep recovery test were fitted to the Burger’s or four element model as per Eq. (8) using the software SIGMAPLOTÒ. The repeated creep recover test measures the total accumulated permanent strain in the binder after the 52 cycles, taking into consideration the elastic recovery after each cycle. The time and strain data were taken towards
S. Biro et al. / Construction and Building Materials 23 (2009) 2080–2086
the end of the test so that the binders have attained a steady state. The ZSVs calculated from the repeated creep recovery test using the Burger’s model are shown in Fig. 8. It can be seen from Fig. 8 that the five binders showed a similar trend as in case with other calculation methods.
cðtÞ ¼
s0 G0
þ
s0 G1
tG1 s0 1 e g1 þ t
ð8Þ
g0
where c is deformation, s0 is shear stress value before the strain step (in relaxation), G0 and G1 are shear modulus values of the springs, g1 is shear viscosity of an individual Maxwell or Kelvin– Voigt element, g0 is zero shear viscosity, t is testing time (s). 3.4. Comparison of different calculation methods From the results obtained in this study, it can be observed that the method of test to calculate the ZSV has a significant effect, however, ZSV values obtained from two different models using the data of a particular test are very similar. The determination of the ZSV from the creep test and the frequency sweep test (oscillation test) are described as the two most commonly used methods of calculating the ZSV of the binders by several researchers [5,6,20,21]. Since the ZSV determined by the creep test and the oscillation test theoretically measure the same property of the binder, the results have been compared by several researchers [7,25,27] and it was found that both the tests gave the same results for unmodified binders and binders with a low polymer content. However, the results of the two tests vary, especially for highly modified binders, because either the steady state has not reached in the binder within the testing duration of the creep test, or the frequency is not sufficiently low to obtain the lower frequency plateau in the viscosity curve in the oscillation test [19]. It was also reported that the Carreau’s and the Cross’ models were not suitable in predicting the ZSV of modified binders, as their curves do not reach a plateau at low shear rate or frequencies. In this study, it was observed that the ZSV values calculated form the creep and the frequency sweep tests were significantly different. The reason for this could be that the binders did not reach a steady state during the creep test, or the frequency of testing during the frequency sweep test was not low enough to obtain a plateau. Since in this study, a stress of 100 Pa was applied for duration of 4 h in the creep test on a binder not modified by any polymer, it is unlikely that the binder did not attain a steady state
1200 Original binder Original binder + Asphamin Original binder + Sasobit
Zero shear viscosity, Pas
1000 800 600 400 200 0 1
2
3
4
5
Binder source Fig. 8. ZSV values for the five binders with and without warm asphalt additives calculated from repeated creep recovery test by the Burger’s model.
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during the creep test. However, during the frequency sweep test, the lowest frequency of testing was 0.01 Hz, which may not be low enough to attain the plateau in the viscosity curve. Since the DSR used could not test at any lower frequencies, a higher temperature of testing can be used to calculate the ZSV from a frequency sweep test. It was decided not to test the samples at a higher temperature, as the ZSV is an indication of the rutting potential, and the high pavement service temperatures are about 60 °C. Calculation of the ZSV at temperatures higher than 60 °C may not have any significance if the pavement is not exposed to such temperatures in the field. Additionally, the authors believe that fitting data from a viscosity master curve can provide better results, as it can provide viscosity data at very low frequencies. Though the results of the ZSV obtained from the creep and frequency sweep tests were different, the trends observed with all the binders were very similar. Also, many factors such as creep testing time, frequency of loading, temperature of testing, software used to fit the test data to calculate the ZSV values, number of iterations used to fit the data, and many other factors affect the value of the ZSV calculated from any model using any test method. ZSV is only a theoretical concept, and there is no way to measure the absolute value, since there are so many approximations, and assumptions involved with the calculation of the ZSV. Thus, it is the opinion of the authors that the ZSV should be used only as a means of observing trends in the binders as a result of modification. In this study, it was observed that the addition of the warm asphalt additives increased the ZSV of the binder, irrespective of the test method and model used, which is consistent with the findings of another preliminary study at Clemson University, where the addition of the warm asphalt additives increased the stiffness of the binders and reduced the permanent deformation of the binders.
4. Conclusions From this limited study, the following can be concluded. It was observed that the addition of the warm asphalt additives increased the ZSV values of the five binders used in this study at 60 °C. In particular, the addition of SasobitÒ increased the ZSV of the binders the most in majority of the cases. This observation is consistent with an earlier study by the authors, where the addition of the warm asphalt additives increased the stiffness of the binders at 60 °C. It is believed that the increase in the ZSV values of the binders due to the addition of AsphaminÒ is as a result of the mineral filling effect of the zeolite, which after initial foaming remains un-dissolved in the binders. Similarly, it is hypothesized that when SasobitÒ is added to the binders, the paraffinic wax is in crystalline state at 60 °C, thereby increasing the stiffness of the binders, and thus the ZSV of the binders at 60 °C. ZSV values calculated from any particular test (creep, frequency sweep test, etc.) using different models are significantly similar to each other. ZSV values calculated from the same model using different test methods are significantly different. Thus, it can be seen that the test method used to calculate the ZSV, and the testing parameters used are important factors in calculating the ZSV of binders. Several factors like the type of test, test parameters used, software used to fit the test data to models, number of iterations performed to fit the test data, etc. affect the ZSV values calculated. Also, several assumptions and approximations are made in order to calculate the ZSV values for binders. As a result of this, the true ZSV value may never be obtained. Thus, the authors feel that the ZSV values should be used only as a means of observing trends in the binders as a result of modification.
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